"Proč je tak malá?"

Translation: Why is she so small?

[FQGfV]

Why is "why is he so small" marked as a typo ( (s)he )?

[AgnusOinas]

Because only "she" is correct - "malá" is feminine.

"Why is he so small?" is just wrong here, Duo mistakenly considers that a typo.

It could also be "Why is it so small?" if you're talking about an item that is feminine in gender.